Microdisplacement problem
You can add the rocks hightmap to your Main hightmap, with a mix node (set to add, factor of 1), or a silmple math add node. Plug the output of the add node into the displacement socket. To control the strength of your displacement use a multiply node inbetween. (Values below 1 will reduce terrain hight, values above 1 will grow it.)
The slope information is only used for color, don't add that to the displacement. It will give you wrong results.